Differential time television system

ABSTRACT

A system for providing a signal which represents the change or displacement in an event or occurrence in a given time period wherein the reference image is continuously updated. In one embodiment, the output from a television system is stored in such a manner that it may be retrieved to provide a pair of output signals. One of the retrieved output signals represents the occurrence of an event at a given time, while the other output signal, which is simultaneously retrieved with the first output signal, represents the occurrence of the event at a time different from the time of the occurrence of the even indicated by the first output. Means are disclosed for subtracting the two retrieved output signals to provide a differential output signal which represents the change in the event during the time corresponding to the difference in time of the occurrence represented by the first retrieved output signal and the second retrieved output signal. In a preferred embodiment, a television camera provides an output signal which is recorded on both sides of a video recording disc. Playback heads, capable of adjustment relative to each other, are offset relative to the registration of the record heads so that the output signals from the first playback head and the second playback head, when subtracted, effectively represent the change in the event over the period of time determined by the playback head offset. Means are disclosed for adjusting the relative positions of the playback heads. Auxiliary means are also disclosed for monitoring the output of the playback head and recording the monitored output. Means for monitoring the output from the differential amplifier and recording that output with a video tape recorder are also disclosed so that the image difference may be added to the background image. Means for summing the output of the video tape recorder together with the recorded signals from the differential amplifier produce an image which has the background information of the event, but wherein the change in the event over a given time period is enhanced. An integrator camera is also disclosed for integrating the &#39;&#39;&#39;&#39;snow&#39;&#39;&#39;&#39; noise out of the television signal. Application of such a system to an X-ray or a fluoroscopic system for the treatment of patients is also disclosed.

United States Patent Corrigan, Jr., deceased 1 July 25,1972

[54] DIFFERENTIAL TIME TELEVISION SYSTEM [72] lnventor: Matthew J. Corrigan, Jr., deceased, late of Willoughby, Ohio by Sandra Lee Corrigan, executrix Electronic Products Corporation, Cleveland, Ohio 22 Filed: Nov. 9, 1970 21 Appl.No.: 88,027

[73] Assignee:

[52] US. Cl ..178/6.6 DD, l78/DIG. 5, l78/DIG. 33, 178/DlG. 37 [51] Int. Cl. ..l-l04n 5/78, H04n 7/02, H04n 7/18 [58] Field of Search ..l79/6.6 A, 6.7 R, DIG. 5, DIG. 21, l79/DlG. 33, DIG. 37, 6.6 DD

Primary Examiner-Howard W. Britton Attorney-Fay, Sharpe & Mulholland 57 ABSTRACT 'A system for providing a signal which represents the change or displacement in an event or occurrence in a given time period wherein the reference image is continuously updated. In one l8 l2 RECORD MEANS TELEVISION CA M 5 RA RE ORD PLAYBACK IO MEANS embodiment, the output from a television system is stored in such a manner that it may be retrieved to provide a pair of output signals. One of the retrieved output signals represents the occurrence of an event at a given time, while the other output signal, which is simultaneously retrieved with the first output signal, represents the occurrence of the event at a time different from the time of the occurrence of the even indicated by the first output. Means are disclosed for subtracting the two retrieved output signals to provide a differential output signal which represents the change in the event during the time corresponding to the difference in time of the occurrence represented by the first retrieved output signal and the second retrieved output signal. In a preferred embodiment, a television camera provides an output signal which is recorded on both sides of a video recording disc. Playback heads, capable of adjustment relative to each other, are ofiset relative to the registration of the record heads so that the output signals from the first playback head and the second playback head, when subtracted, effectively represent the change in the event over the period of time determined by the playback head oflset. Means are disclosed for adjusting the relative positions of the playback heads. Auxiliary means are also disclosed for monitoring the output of the playback head and recording the monitored output. Means for monitoring the output from the differential amplifier and recording that output with a video tape recorder are also disclosed so that the image difference may be added to the background image. Means for summing the output of the video tape recorder together with the recorded signals from the differential amplifier produce an image which has the background information of the event, but wherein the change in the event over a given time period is enhanced. An integrator camera is also disclosed for integrating the snow" noise out of the television signal. Application of such a system to an X-ray or a fluoroscopic system for the treatment of patients is also disclosed.

23 Claims, 5 Drawing figures v10 E0 MONITOR TA P E RECORDER DIFFERENTIAL 3 AMPLIFIER 7 29 7a MONITOR VTIADPEEO RECORDER 50 m TE e RATO R CAM ERA L53 MON lTOR PATENTEnJuL 2 5 m2 SHEET 2 [1F 2 IMAGE INTENSIFI ER :PATI ENT I X-RAY UN IT mvsmon MATTHEW J. CORRIGAN,JI'.

. DECESED DIFFERENTIAL TIME TELEVISION SYSTEM BACKGROUND OF THE INVENTION This invention relates to a system for providing an output which represents the change in an event during a selectably variable time period in which the reference datum for the event is continuously updated. More particularly, this invention relates to a system for the simultaneous retrieval of stored information representing the occurrence of an event in such a manner that the first retrieval represents the occurrence of the event at a first time and a second retrieval represents the occurrence of an event at a second time. Still more particularly, this invention relates to a television system in circuit with means for subtracting a first output signal indicative of an event at a first time from a second output signal indicative of the event at a second time to produce a television image which re resents the change in the event between the two times, and in which the reference image is continuously updated.

It is a problem in this art to observe the changes in an event over a given period of time. Perhaps the best known technique is the stroboscopic pulsing of light upon a moving object at regularly-timed intervals which has the visual effect of achieving a stop action of the moving object so that its characteristics may be observed. When used with appropriately timed photographic devices, the stroboscopic technique is effective in stopping and retaining the action of an observed event. However, there are a large number of events, for example, the passage of a radiologically opaque fluid in the body, which are not readily susceptible to the use of stroboscopic techniques.

Moreover, in certain instances, it is desired to have an instant observation of the change in the event over a certain time period. This aim is not readily achievable with conventional camera techniques because of the length of time for developing the film. Furthermore, there is no truly effective photographic technique for immediately comparing the two images derived over a given time period. Such a process generally depends upon the acuity of the observer or an effective overlay of the photonegatives. When the overlay technique is used, a precise determination of the change in the event for a given period is difficult because of the background material which is necessarily present with such techniques. Accordingly, it is an object of the invention to provide a system which is effective for determining the change in an event over a given time period and which overcomes the shortcomings of the aforementioned prior art.

There are a number of comparative techniques known to the art which are capable of determining the change in an event over a given period of time, but, in general, such techniques have not been entirely satisfactory because of their inability to update the referenced image. For example, in observing the subtle changes in the operation of a machine over an operative cycle, it may well be that the critical changes in the machine occur between the fifth and sixth time increment of the cycle. Present techniques are largely confined, however, to referencing the observed image against a datum image which was sensed at time zero.

Another complication in the techniques thus described is the possibility of motion in the background. In such occasions, it becomes increasingly difiicult to correlate the images, a particular disadvantage when the change in a short period in the midst of a cycle is desired to be observed. Thus, it is also an aim of the art to provide a method for televising such events and utilizing television techniques to observe the changes in an event over a given period of time.

In the field of radiology, television techniques have been particularly interesting and advantageous to store for later observation the images provided by an X-ray fluoroscopy system. For example, a system is known which stores images on a disc recorder in which the image is processed by subtraction to find the change in the radiological event over a given time period. In this process, two radiographs are made, the first without the radio opaque substance in the body of the patient, and the second with the radio opaque substance within the body of the patient. The two radiographs are compared in a device which is capable of cancelling out the background material provided that there is perfect position registration. Such a system uses a differential amplifier to compare one recorded frame of information with another so that the background effects, such as may be caused by slight and minor movement of the patient, are not in large measure eliminated.

Such a technique is not altogether satisfactory because of its inability to update the datum signal during the course of the examination. More precisely, it may be of critical significance to the radiologist or the examining physician that the passage of the opaque fluid through a constricted portion of an artery or a vein, for example, may occur between the fifth and sixth time period of the fluoroscopic examination. Thus, it would be advantageous to provide a television technique which is able to compare the image at the sixth portion of the cycle with the image observed at the end of the fifth portion of the cycle. Thus, it is an object of this invention to provide a television system which utilizes subtraction techniques in which the referenced image is continuously updated.

When two television cameras are used to record separately the image obtained during the examination, another problem arises in that the separate output signals must be effectively registered and synchronized, a technique which is quite difficult with the current state of the art. Thus, it is another object of this invention to provide a television system for the recording of the change in an event over a given time period where the datum image is continuously updated and which utilizes a single source of signals.

These and other objects of the invention will become apparent upon a review of this specification in light of the accompanying drawings.

BRIEF SUMMARY OF THE INVENTION Directed to achieving the objects stated above and overcoming the shortcomings of the prior art, this invention relates to a system which is capable of automatically and instantaneously representing the change in an event during a given time period where the datum image is continuously updated. The signals from a suitable source are stored in such a manner that they may be simultaneously retrieved by two means for retrieving the signals which are offset in time from each other. The output signals from the retrieving means provide the inputs to a differential amplifier having an output which represents the change in the event over the period of time under scrutiny. When the retrieved signals represent occurrences which are, for example, one-sixth of a second apart, although retrieved simultaneously, it may be seen that the output from the differential amplifier represents the change in the event over that one-sixth of a second period.

In a preferred embodiment, a television camera provides the input signal, which may be the output, for example, of a fluoroscopic X-ray unit. The signals from the television camera are recorded on the upper and lower surfaces of a video disc by record heads which are in register. The playback heads, which physically may also be the record heads, are offset one from the other so that during the playback mode, the signals retrieved from the recorded disc are offset in time but retrieved simultaneously. Means are disclosed for adjusting the positioning of the playback heads so that the differential between the two signals may be adjusted. The output of the differential amplifier is connected to a monitor which displays the change of the event with the coherent information between the two retrieved signals eliminated.

It is an additional feature of the invention to monitor and record one of the retrieved signals from the disc and to add it to the enhanced image of the differential signal, so that the enhanced differential image is displayed against the background or environment of the event. Such a technique, for example, would be desirable for an examining physician to orient the passage of a radio-opaque fluid through a vein by orienting it against a background of ribs, organs and the like.

It is another feature of the invention to utilize an integrator camera for observing the image output from the differential amplifier. The integrator camera is capable of integrating out the high frequency incoherent signals, such as snow noise, which vary at a high rate with respect to time. The integrator camera takes the differential time segments and reassembles the total image, such as a vascular system, in its true shape or form with the background, ribs and the like, eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a block diagram of one embodiment of the system according to the invention;

FIG. 2 is a top plan view of a video recording disc showing one arrangement which has the record heads in register and the playback heads in an offset relationship, one of the playback heads being set forth in phantom outline;

FIG. 3 is a side view of the video disc, recording heads and playback heads shown in FIG. 2;

FIG. 4 is an alternative input arrangement in which the television camera observes as its input signal the output from a fluoroscopic unit; and,

FIG. 5 is a diagrammatic view of the passage of an opaque fluid through a vein in the body of a patient showing several images which may be obtained with the system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, means are shown generally at for providing an output signal which represents an event observed over a given period of time. The input means 10 comprise a television camera 11 which provides an output signal on lead 12 for storage by recording means, designated generally at 14, in such a manner that the signals representing the event at sequential periods of time may be simultaneously recorded. Thus, in the embodiment shown, a video recording disc 16 stores the output from the television camera 11 by a first record means 17 and a second record means 18 which may be disposed above and below the recording surfaces of the disc 16. Preferably, the record means are record heads.

Means for retrieving the stored signal are designated generally at 20 and comprise a first playback head 21 and a second playback head 22, disposed on opposite sides of the video disc in a manner similar to the record heads 17 and 18. However, the playback heads are offset one from the other by an amount determined by the adjustment of the head positioning means 24. Thus, in this manner, the operation of the retrieving means 20 permits a simultaneous retrieval of the signals stored on the disc 16 by playback heads 21 and 22 which thus represent the event as it occurred at two distinct periods of time. Accordingly, the reference image is continuously updated. The output signal from playback head 21 is provided by lead 26 which is connected to a switch 27 which, in the position shown, permits the input to be provided to a differential amplifier 29. It should be understood, however, that it is not necessary to have separate record and playback heads to practice the invention.

The output from playback head 22 is provided on lead 30 to a switch 31 which, in the position shown, permits the input to be provided to a differential amplifier 29.

The respective images provided on leads 33 and 26 respectively are effectively subtracted in the differential amplifier 29 to provide an output on lead 35 which represents the difference between the two input images. In this manner, the output of the differential amplifier presents an enhanced image of the change in the televised occurrence over a given period of time in which the datum reference level for the differential amplifier is continuously updated.

The signals recorded by the television camera 11 may be considered to include three time components. First, background information, such as bone detail and the like, framing and so forth, may be effectively considered not to vary with time during the recording of the event. A second portion of the information observed by the television camera is that information which is of interest to the observer and which varies at a relatively fast rate with respect to time. An example is the passage of the radio-opaque fluid through a vein in the body. A third time component of the television signal is the incoherent noise which varies at a very high rate relative to the coherent signals and with respect to time and is normally referred to in the trade as snow". Thus, the output of the differential amplifier 29 will eliminate the coherent portion of the signal, or the background, and will emphasize the difference in recorded information over a given period of time. Moreover, the differential amplifier may have some effect on the incoherent noise, a factor which may be compensated for as will be described later in the specification. Thus, the events as observed on monitor 38 connected to lead 35 with switches 27 and 31 positioned as shown emphasize the occurrence which is of interest to the observer.

A second feature of the invention rests in means to display the output of the playback head 22 on the lead 30 on a monitor 40 connected to the lead 30 by a lead 41. The monitored image shows the occurrence substantially as it is observed. The output of the instantaneous recording displayed on the monitor 40 is recorded on video tape recorder 43 connected by lead 44 to the monitor 40.

Similarly, the monitored event, primarily represented by the enhanced differential signal may be recorded on a video tape recorder 46 connected to the monitor 38 by a lead 47. When operated in this fashion, the signals recorded on the tape recorder 46 represent the sum of the images of the subtraction. The output of the video tape recorder 46 is connected by lead 48 to an adding input of the differential amplifier 29 when switch 27 is in its alternative position. The output of the video tape recorder 43 is connected by lead 49 to a second adding input of the differential amplifier when switch 31 is in its second position. The differential amplifier is thus also capable of adding the signals which appear on leads 48 and 49 so that, when the respective signals are in register, the event recorded when the switches 27 and 31 are respectively in their second positions is a reconstructed image of the event showing an enhanced signal representing the slowly varying information in its true background or environment.

The capability of the system to reconstruct the background of the observed occurrence is particularly advantageous in orienting the occurrence of the event in the body of a patient, for example.

The incoherent noise may be eliminated by observing the reconstructed image on monitor 38 by an integrator camera 50 which is capable of integrating the incoherent noise signal to some suitable D.C. level to further enhance the reconstructed image. The output of the integrator camera 50 is connected by a lead 52 to a monitor 53 where it may be instantaneously observed. The output of the monitor 53 is connected by lead 54 to a video tape recorder 55 in which the reconstructed image having an enhanced image of interest may be recorded for future reference.

In FIG. 2, a generally circular television recording disc is designated generally at 60 and includes a plurality of concentric recording tracks radially disposed from the center 61 of the disc. In the top view shown in FIG. 2, the top record head 18 having an input lead 19 is shown. The top playback head 22 is shown with its output lead 30. The record and playback heads may be offset relative to each other at any desirable position with respect to the recording disc 60, such as at 90 or I". The second playback head 21 is shown in phantom outline to illustrate its offset from the first playback head 22 during the playback process. Since the record head 17 is in register with record head 18, it is not shown in FIG. 2.

a In FIG. 3, means are designated generally at 70 for adjusting the relative offset between the playback heads 21 and 22 to control the time differential of the signal of interest to the observer. In the embodiment shown, the offset is achieved by mechanical linkages.

The offset adjusting means 70 comprises a member 71 hav ing a pair of openings disposed therein for receiving members 72 and 73 therein. Member 72 is fixedly secured to the playback head 22 and to member 71, while member 73 is fixedly secured to the playback head 21 and adjustably secured to member 73. A releasable member, such as a set screw 75, is axially disposed in member 71 for releasably securing fixed member 73 so that the offset of the playback head 21 may be adjusted relative to the position of the playback head 22 on the opposed side of the recording disc 60.

In operation, the signals from the television camera 11 are recorded simultaneously on the opposite sides of disc 60 by the record heads 17 and 18 which are positioned in register. Either simultaneously with the recording of the event, or when desired, the recorded signals are retrieved by the playback heads 21 and 22. Thus, while the signals retrieved by playback heads 21 and 22 are obtained at the same time, the outputs thereof represent the occurrence of an event at two distinct periods of time wherein the reference image is continuously updated. This provides a significant advantage over systems which do not have a continuous update capability.

FIG. 4 shows a partial block diagram in which the signals observed by the television camera 11 are obtained from a fluoroscopic unit designated generally at 80 having an image intensification capability.

The fluoroscopic unit 80 comprises an X-ray unit 81 for observing a patient 82. The output of the X-ray unit 81 is passed through the patient 82 to an image intensifier unit 85. The output of the image intensifier unit is recorded by the television camera and the operation of the system proceeds in the manner previously described.

In FIGS. 5a5c, the representation of an artery in the body I is designated at 88 having radio-opaque fluid 89 passing therethrough. By way of background, FIG. 5a shows a vein 90 and a rib 91 which comprise background information received by the television camera from the embodiment in FIG. 4. In FIG. 5a, at the end of a fifth time period, for example, it may be seen that the radio-opaque fluid has progressed to a point defined by the interface 92 between the radio-opaque fluid 89 and the blood 93 in the artery. At a later time, determined by the offset of the playback head as indicated in FIG. 3, the radio-opaque fluid has progressed to a point defined by the interface 94 between the radio-opaque fluid 89 and the blood 93. When the signals are provided to the differential amplifier 29, as described in connection with FIG. 1, the enhanced signal is that shown in FIG. 50, which illustrates only the passage of the radio-opaque fluid 89 for the time period under scrutiny from the interface 92 to the interface 94. Similarly, the coherent background information, such as would be represented by the vein 90 and the rib 91, has been eliminated in a manner previously described.

The reconstruction function as previously described in connection with FIG. 1 has the capability of again providing the background information, for example, the rib 91 and the image 90, by the adding feature previously described so that the enhanced image illustrated in FIG. 50 may be observed against its true background of rib 91 and vein 90.

By way of an example, a video recording disc is generally designed to operate at thirty cycles per second so that each revolution of the disc represents one-thirtieth of a second. If the offset of the playback heads is adjusted so that five frames, or revolutions of the disc, are disposed between the relative positions of the playback heads 21 and 22, it may be seen that the image displayed by the differential amplifier in the first instance represents an elapsed time of one-sixth of a second. Thus, the system is capable of observing other timed events, such as blood velocity, as well. It should be noted that the difference may be observed between the 275th and 271st frame or between the 90th and the 93rd frame, or in any combination of frames desired.

The system described also has distinct advantages in compensating for the movement of the patient during the action of the X-ray unit 81 because of its subtraction capabilities.

While the preferred embodiment has been described in conjunction with a video disc recorder in which the images are stored above and below on the upper and lower surfaces of the disc, the images may be stored in any electronic means which is capable of providing a simultaneous retrieval of the information which represents the occurrence of the event at different periods, such as wire recorders, memory banks and the like. For example, it is contemplated that in certain applications of the invention, only a single record head will be needed to store the information in a recording means so long as the recording means is capable of providing playback and simultaneous offset playback in the manner described.

The invention could also be used in industrial applications to observe subtle variations in machinery with respect to an increment of time or to observe the changes in the material or product produced by that machinery.

The invention may be also embodied in other specific forms without departing from its spirit or essential characteristics. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the claims rather than by the foregoing description, and all changes which come within the meaning and range of the equivalents of the claims are therefore intended to be embraced therein.

What is claimed is:

1. An electrical-mechanical system for recording and retrieving electrical analog representations in real time of the occurrence of an event comprising:

first means for storing in real time a retrievable electrical analogue representation of the occurrence of said event for a discrete continuous period of time, said representation being characterized as being retrievable in real time to provide at least a pair of outputs indicating the status of said event at a selected predetermined time interval defined by the difference between a terminal time and a datum reference time within said discrete continuous period of time;

second means for retrieving said stored electrical analogue representation in real time and for producing a first output indicative thereof, said first output indicating the occurrence of said event at said datum reference time within said discrete continuous period of time, said datum reference time being continuously updated during the retrieval of said representation by said second means; third means for retrieving said representation in real time simultaneously with the retrieval of said representation by said second means, characterized in that the representation of said event retrieved by said third means is displaced in time for a period equal to said selected time interval from the datum reference time representation retrieved by said second means, and for producing a second output indicative thereof, said second output indicating the occurrence of said event at the terminal time of said predetermined time interval, the end of said predetermined time interval being continuously updated during the retrieval of said representation by said third means;

fourth electrical circuit means for determining the difference between said second output and said first output and for providing a third output indicative of said difference, whereby the third output automatically and continuously represents the change in the event during the time corresponding to said selected predetermined time interval; and

fifth electrical means for translating said third output into an observable image which represents the change in the event during the time corresponding to said selected predetermined time interval.

2. The system as defined in claim 1 further including means for selecting the displacement in time between the representations respectively retrieved by said second means and said third means.

3. The system as defined in claim 1 wherein said fourth electrical circuit means is a differential amplifier including a first input circuit coupled to said second means for receiving said first output thereof, a second input circuit coupled to said third means for receiving said second output thereof and an output circuit for providing the output signal representing an enhanced difference between said first input and said second input.

4. The system as defined in claim 1 further including sixth means for storing said first output;

seventh means for storing said third output;

eighth means for retrieving said first output;

ninth means for retrieving said third output at the same time that said eighth means retrieves said first output; and

tenth means for combining the retrieved first output and the retrieved third output to assemble the difference signal on a background representative of the true background of the event.

5. The apparatus as defined in claim 1 further including means for monitoring said first output and means for monitoring said third output.

6. The system as defined in claim 1 further including means for integrating the third output signal.

7. The system as defined in claim 6 further including means for recording said integrated signal.

8. The system as set forth in claim 1 wherein the event recorded and retrieved is the passage of an opaque fluid through a blood vessel and said system further includes means for observing the passage of said opaque fluid in real time and providing an output signal in real time indicative thereof, wherein the change in the passage of said fluid through said blood vessel is produced and the coherent background of bones, blood vessels and the like is eliminated.

9. The system as set forth in claim 8 further including means for assembling the enhanced-difference signal representing the passage of the fluid through said blood vessel on the background representative of the true background of the event including the presence of bones, blood vessels and the like.

10. A system as defined in claim 1 further including input means for observing said event in real time providing an output signal in real time indicative thereof.

11. The system as defined in claim 3 wherein said first means is a recording means capable of recording said output signal in such a manner that the recorded output signal may be simultaneously retrieved by said second means and said third means to produce said first output and said second output.

12. The system as defined in claim 11 wherein said recording means is a video disc recorder and further includes:

a first record head for recording said output signal at a first location on said video recording disc;

a second record head for recording said output signal at a second location on said video recording disc;

a first playback head for retrieving the output signal from said first location on said video recording disc; and,

a second playback head for retrieving the output signal from said second location on said video recording disc at the same time that said signals are retrieved by said first playback head and wherein the retrieved signal from said second location on said video disc represents the occurrence of the event displaced in time from the occurrence represented by the retrieved signal from said first location.

13. The system as defined in claim 12 further including means for adjusting the position of said second playback head relative to said first playback head wherein said recording means is a video disc and said first location is on said. one side of said disc, said second location is on the other side of said disc, said first playback head is on said one side of said disc, and said second playback head is on said second side of said disc.

14. A method of recording and retrieving electrical analog representations in real time of the occurrence of an event and providing an output which represents the change in said event over a selected predetermined time interval defined by the difference between a terminal time and a datum reference time further characterized in that the reference signal is continuously updated comprising the steps of:

storing in real time a retrievable electrical analogue representation of the occurrence of said event for a discrete continuous period of time, said representation being characterized as being retrievable in real time to provide at least a pair of outputs indicating the status of said event at said selected predetermined time interval within a discrete continuous period of time;

retrieving said stored electrical analogue representation in real time and producing a first output indicative of the event at said datum reference time, said first output in dicating the occurrence of said event at said datum reference time within said discrete continuous period of time, said datum reference time being continuously updated during the retrieval of said representation by said second means;

retrieving said representation in real time simultaneously with the said step of retrieving and producing said first output characterized in that the representation of said event is retrieved in real time for a period equal to said selected time interval from said datum reference time and producing a second output indicative thereof;

electrically determining by electrical circuit means the difference between the second output and the first output and providing a third output indicative thereof; and translating the third output into an observable image which represents the change in the event during the time corresponding to said selected predetermined time interval.

15. The method as defined in claim 14 further including the step of selecting the displacement in time between the representation respectively retrieved by said retrieving steps.

16. The method as defined in claim 14 further including the steps of storing said first output, storing said third output, retrieving the stored first output, retrieving the stored third output and combining the retrieved first output and third output to construct a composite signal having an enhanced representation of the change in the event over the time period corresponding to said displaced time.

17. The method as defined in claim 14 further including the step of monitoring said first output'and monitoring said third output.

18. The method as defined in claim 14 further comprising the step of integrating said third output to reduce high frequency components thereof.

19. The method as defined in claim 18 further including the step of recording the integrated output.

20. The system as defined in claim 14 further including the step of observing said event in real time and providing a composite output signal in real time indicative thereof.

21. The method as defined in claim 20 wherein the step of recording is further defined as the step of video disc recording and further includes the steps of recording said output signal at a first location on a video recording disc, recording said out- .put signal at a second location on the video recording disc,

retrieving the output signal from said first location on said video recording disc, and retrieving said output signal from said second location on said video recording disc simultaneously with the retrieval of said output signal from said first location and wherein the retrieved signal from said second location is displaced in time from the retrieved signal from the first location.

22. The method as set forth in claim 14 wherein the steps of recording and retrieval are further limited to recording and retrieving a representation of the passage of an opaque fluid through a blood vessel and the method further includes the steps of observing the passage of said opaque fluid in real time and providing an output signal in real time indicative thereof, wherein the change in the passage of said fluid through said blood vessel is produced and the coherent background of bones, blood vessels and the like is eliminated.

23. The method as set forth in claim 22 further including the step of assembling the enhanced difference signal representing like. 

1. An electrical-mechanical system for recording and retrieving electrical analog representations in real time of the occurrence of an event comprising: first means for storing in real time a retrievable electrical analogue representation of the occurrence of said event for a discrete continuous period of time, said representation being characterized as being retrievable in real time to provide at least a pair of outputs indicating the status of said event at a selected predetermined time interval defined by the difference between a terminal time and a datum reference time within said discrete continuous period of time; second means for retrieving said stored electrical analogue representation in real time and for producing a first output indicative thereof, said first output indicating the occurrence of said event at said datum reference time within said discrete continuous period of time, said datum reference time being continuously updated during the retrieval of said representation by said second means; third means for retrieving said representation in real time simultaneously with the retrieval of said representation by said second means, characterized in that the representation of said event retrieved by said third means is displaced in time for a period equal to said selected time interval from the datum reference time representation retrieved by said second means, and for producing a second output indicative thereof, said second output indicating the occurrence of said event at the terminal time of said predetermined time interval, the end of said predetermined time interval being continuously updated during the retrieval of said representation by said third means; fourth electrical circuit means for determining the difference between said second output and said first output and for providing a third output indicative of said difference, whereby the third output automatically and continuously represents the change in the event during the time corresponding to said selected predetermined time interval; and fifth electrical means for translating said third output into an observable image which represents the change in the event during the time corresponding to said selected predetermined time interval.
 2. The system as defined in claim 1 further including means for selecting the displacement in time between the representations respectively retrieved by said second means and said third means.
 3. The system as defined in claim 1 wherein said fourth electrical circuit means is a differentiAl amplifier including a first input circuit coupled to said second means for receiving said first output thereof, a second input circuit coupled to said third means for receiving said second output thereof and an output circuit for providing the output signal representing an enhanced difference between said first input and said second input.
 4. The system as defined in claim 1 further including sixth means for storing said first output; seventh means for storing said third output; eighth means for retrieving said first output; ninth means for retrieving said third output at the same time that said eighth means retrieves said first output; and tenth means for combining the retrieved first output and the retrieved third output to assemble the difference signal on a background representative of the true background of the event.
 5. The apparatus as defined in claim 1 further including means for monitoring said first output and means for monitoring said third output.
 6. The system as defined in claim 1 further including means for integrating the third output signal.
 7. The system as defined in claim 6 further including means for recording said integrated signal.
 8. The system as set forth in claim 1 wherein the event recorded and retrieved is the passage of an opaque fluid through a blood vessel and said system further includes means for observing the passage of said opaque fluid in real time and providing an output signal in real time indicative thereof, wherein the change in the passage of said fluid through said blood vessel is produced and the coherent background of bones, blood vessels and the like is eliminated.
 9. The system as set forth in claim 8 further including means for assembling the enhanced difference signal representing the passage of the fluid through said blood vessel on the background representative of the true background of the event including the presence of bones, blood vessels and the like.
 10. A system as defined in claim 1 further including input means for observing said event in real time providing an output signal in real time indicative thereof.
 11. The system as defined in claim 3 wherein said first means is a recording means capable of recording said output signal in such a manner that the recorded output signal may be simultaneously retrieved by said second means and said third means to produce said first output and said second output.
 12. The system as defined in claim 11 wherein said recording means is a video disc recorder and further includes: a first record head for recording said output signal at a first location on said video recording disc; a second record head for recording said output signal at a second location on said video recording disc; a first playback head for retrieving the output signal from said first location on said video recording disc; and, a second playback head for retrieving the output signal from said second location on said video recording disc at the same time that said signals are retrieved by said first playback head and wherein the retrieved signal from said second location on said video disc represents the occurrence of the event displaced in time from the occurrence represented by the retrieved signal from said first location.
 13. The system as defined in claim 12 further including means for adjusting the position of said second playback head relative to said first playback head wherein said recording means is a video disc and said first location is on said one side of said disc, said second location is on the other side of said disc, said first playback head is on said one side of said disc, and said second playback head is on said second side of said disc.
 14. A method of recording and retrieving electrical analog representations in real time of the occurrence of an event and providing an output which represents the change in said event over a selected predetermined time interval defined by the difference between a terminal time and a datum referEnce time further characterized in that the reference signal is continuously updated comprising the steps of: storing in real time a retrievable electrical analogue representation of the occurrence of said event for a discrete continuous period of time, said representation being characterized as being retrievable in real time to provide at least a pair of outputs indicating the status of said event at said selected predetermined time interval within a discrete continuous period of time; retrieving said stored electrical analogue representation in real time and producing a first output indicative of the event at said datum reference time, said first output indicating the occurrence of said event at said datum reference time within said discrete continuous period of time, said datum reference time being continuously updated during the retrieval of said representation by said second means; retrieving said representation in real time simultaneously with the said step of retrieving and producing said first output characterized in that the representation of said event is retrieved in real time for a period equal to said selected time interval from said datum reference time and producing a second output indicative thereof; electrically determining by electrical circuit means the difference between the second output and the first output and providing a third output indicative thereof; and translating the third output into an observable image which represents the change in the event during the time corresponding to said selected predetermined time interval.
 15. The method as defined in claim 14 further including the step of selecting the displacement in time between the representation respectively retrieved by said retrieving steps.
 16. The method as defined in claim 14 further including the steps of storing said first output, storing said third output, retrieving the stored first output, retrieving the stored third output and combining the retrieved first output and third output to construct a composite signal having an enhanced representation of the change in the event over the time period corresponding to said displaced time.
 17. The method as defined in claim 14 further including the step of monitoring said first output and monitoring said third output.
 18. The method as defined in claim 14 further comprising the step of integrating said third output to reduce high frequency components thereof.
 19. The method as defined in claim 18 further including the step of recording the integrated output.
 20. The system as defined in claim 14 further including the step of observing said event in real time and providing a composite output signal in real time indicative thereof.
 21. The method as defined in claim 20 wherein the step of recording is further defined as the step of video disc recording and further includes the steps of recording said output signal at a first location on a video recording disc, recording said output signal at a second location on the video recording disc, retrieving the output signal from said first location on said video recording disc, and retrieving said output signal from said second location on said video recording disc simultaneously with the retrieval of said output signal from said first location and wherein the retrieved signal from said second location is displaced in time from the retrieved signal from the first location.
 22. The method as set forth in claim 14 wherein the steps of recording and retrieval are further limited to recording and retrieving a representation of the passage of an opaque fluid through a blood vessel and the method further includes the steps of observing the passage of said opaque fluid in real time and providing an output signal in real time indicative thereof, wherein the change in the passage of said fluid through said blood vessel is produced and the coherent background of bones, blood vessels and the like is eliminated.
 23. The method as set forth in claim 22 further including thE step of assembling the enhanced difference signal representing the passage of the fluid through said blood vessel on the background representative of the true background of the event including the presence of bones, blood vessels and the like. 